Glass flakes allow resin moldings to have improved strength and dimensional accuracy when being dispersed, for example, in a resin matrix. These glass flakes are mixed in coating materials as liners and are then applied to metal or concrete surfaces.
Glass flakes exhibit metallic colors when the surfaces thereof are coated with metal. On the other hand, they exhibit interference colors due to interference of reflected light when the surfaces thereof are coated with metal oxides. That is, a glass flake coated with a coating film formed of metal or metal oxide also can be used as a luster pigment.
Luster pigments produced with glass flakes as described above are commonly used for applications such as coating materials and cosmetics where color tone and luster are considered to be important.
JP 63(1988)-201041 A describes, as suitable compositions for glass flakes, compositions of C glass produced with chemical durability being considered to be important, E glass developed for electronic products, and sheet glass.
JP 2001-213639 A describes glass flakes with excellent chemical durability. The glass flakes with excellent chemical durability contain neither diboron trioxide (B2O3) nor fluorine (F), which are volatile components, and the content of alkali metal oxides therein is 5 mass % or lower.
Glass compositions that are not flaky but fibrous with lower contents of alkali metal oxides are disclosed in the following publications:    JP 61(1986)-14152 A: “Glass Fiber”    JP 2001-515448 A: “Boron-Free Glass Fibers”    JP 2003-500330 A: “Glass Fiber Composition”    JP 2004-508265 A: “Glass Fiber Forming Compositions”    JP 2005-506267 A: “Glass Fiber Forming Compositions”
Glass flakes can be produced by using an apparatus described, for example, in JP 5(1993)-826 A. With the apparatus described in the publication, a molten glass base material is blown up into a balloon shape with a blow nozzle to form a hollow glass film, and this hollow glass film is crushed with a pressure roll. Thus glass flakes can be obtained.
When the production processes as described above are taken into consideration, glass flakes are required to have excellent meltability, a suitable temperature-viscosity property, and a lower devitrification temperature than a working temperature. In this case, the working temperature is a temperature at which glass has a viscosity of 1000 dPa·sec (1000 poise). Furthermore, the devitrification temperature is the temperature at which crystals are formed in the molten glass base material and start to grow. As to the temperature-viscosity property, the working temperature is preferably 1300° C. or lower because an excessively high working temperature particularly makes it difficult to form glass flakes. With the decrease in working temperature of the glass, the fuel cost required for melting glass raw materials can be reduced. Moreover, the decreased working temperature reduces the thermal damage to melting furnaces and apparatuses for producing glass flakes, allowing the life cycles of the furnaces and production apparatuses to be extended.
Apparatuses for producing glass flakes using centrifugal force, for example, have structural bodies, such as a cup that rotates at high speed to allow molten glass to flow out. Excessively high temperature of the molten glass deforms components of the structural bodies, leading to possible failure of the production apparatuses. Moreover, the production apparatuses tend to be eroded by the glass, resulting in shorter life cycles of the production apparatuses.
Furthermore, when a coating film made of metal or metal oxide is to be formed on the surface of a glass flake, the glass flake may be treated at a high temperature. Moreover, glass flakes or those with a coating film may be mixed in a coating material, which may be used for a baking finish to be treated at a high temperature, for example. Therefore glass flakes also are required to have a sufficiently high heat resistance.
Soda-lime glass that is used commonly as a so-called sheet glass composition contains a large amount of alkali metal oxides and therefore does not have a sufficiently high heat resistance, which has been a problem.
In the C glass composition and E glass composition among the compositions of the glass flakes described in JP 63(1988)-201041 A, diboron trioxide (B2O3) and fluorine (F) are essential components to be contained to adjust the devitrification temperature and viscosity. However, since diboron trioxide (B2O3) and fluorine (F) tend to volatilize, there is a possibility that they disperse during melting. Moreover, there also is a possibility of causing a problem in that, for example, they may erode the wall of a melting furnace or a regenerative furnace to reduce the furnace life.
Furthermore, in all the examples described in JP 2001-213639 A, glasses always contain any one component selected from zinc oxide (ZnO), barium oxide (BaO), strontium oxide (SrO), and zirconium oxide (ZrO2).
However, since the zinc oxide (ZnO) tends to volatilize, there is a possibility that it disperses during melting. Furthermore, there is also a problem in that since it volatilizes, the content thereof in the glass is difficult to control.
Generally, the raw materials of barium oxide (BaO) are expensive. Some of them require to be handled with care.
The raw materials of strontium oxide (SrO) are expensive. They may contain raw materials of barium oxide (BaO). Therefore some of them require to be handled with care.
The zirconium oxide (ZrO2) increases the devitrification growth rate of glass and thereby often makes it difficult to produce glass flakes stably.
From such reasons as described above, it is desirable not to use diboron trioxide (B2O3), fluorine (F), zinc oxide (ZnO), barium oxide (BaO), strontium oxide (SrO), and zirconium oxide (ZrO2) in glass flakes.
Moreover, when consideration is given to the fact that the glass flakes are to be mixed in coating materials and cosmetics, they are required to have a high chemical durability.